The present invention relates to lightweight resin moldings and to a method for producing them. Precisely, it relates to fiber-containing, lightweight resin moldings with good appearances which, though being lightweight, bear all the characteristics of high stiffness, high flexural strength, high impact strength, good uniformity in such mechanical strength, and high resistance to local stress and distortion, and also relates to an efficient method for producing them.
Heretofore known are fiber-reinforced resin moldings, which are reinforced with fibers such as glass fibers and the like added thereto. As having good mechanical properties of high tensile strength and high flexural strength and having good heat resistance, fiber-reinforced resin moldings are widely used in various fields of, for example, car parts such as in-panel cores, bumper beams, door steps, roof racks, rear quarter panels, air cleaner cases, etc.; chassises for electric appliances such as radios, televisions, videos, etc.; housings and components of computers, printers, duplicators, etc.; parts and members for furniture, buildings and civil engineering construction, such as outer panels, partition panels, shelf boards, cable troughs, etc. For producing such fiber-reinforced resin moldings, employable is an injection-molding method of injecting a fiber-containing resin melt into molds. According to the injection-molding method, even complicated moldings are obtainable. In addition, another advantage of the method is that a large number of moldings all having the same shape can be produced on a mass-production scale since a predetermined molding cycle of the method can be repeated continuously.
To enhance the strength and the stiffness of fiber-reinforced resin moldings produced in such an injection-molding method, the amount of the fibers to be in the moldings may be increased. However, the increase in the fiber content of the moldings results in the increase in the weight of the moldings, often enlarging the degree of warping deformation of the moldings. Therefore, to reduce the weight of the moldings, a foaming agent may be added to the molding materials. A foam injection-molding method of foaming and molding a molding resin material containing a foaming agent has been proposed (see Japanese Patent Laid-Open No. 247679/1995, etc.). In the foam injection-molding method, however, it is not easy to attain a satisfactorily high blow ratio of moldings, even though a relatively large amount of the foaming agent is added to the molding materials so as to reduce the weight of the foamed moldings to a satisfactory degree. In addition, even when a satisfactorily high blow ratio of moldings could be attained in the method, the mold pattern transferability onto the moldings is not so good, and the moldings will have many silver streaks on their surface, and their appearances will be poor. What is more, the moldings could not have high strength and good stiffness.
To solve the problems, some other methods have been proposed for reducing the weight of resin moldings while making them have good mechanical properties of high strength, good stiffness and high impact resistance and have good appearance quality. (1) One is a blow-molding method of producing lightweight moldings, for which are used fiber-reinforced resin pellets that contain relatively long reinforcing fibers. In the method, the resin pellets are molded into lightweight moldings while the resin is expanded by the spring-back function of the fibers. (2) Another is to add a foaming agent to the fiber-reinforced resin pellets in the blow-molding method (1). In this, the foaming agent added promotes the resin expansion, and the weight of the moldings produced could be reduced more. (See PCT/97/29896.) In these methods, the weight of the moldings produced could be well reduced and the mechanical properties of the moldings will not be so much worsened. Therefore, the methods will be effective for reducing the weight of fiber-reinforced resin moldings.
On the other hand, a method of reducing the weight of resin moldings is well known, which comprises introducing a pressure fluid into a mold cavity filled with a resin melt to be molded, and moving a moving mold member in the mold-opening direction to enlarge the mold cavity volume thereby forming a hollow space in the resulting resin molding. In this method, however, the resin melt adjacent to the facing mold member surfaces is stretched and drawn when the moving mold member is moved in the mold-opening direction, and, in addition, the surfaces of the resin molding adjacent to the facing mold members often have a large number of irregular projections owing to the influence thereon of the pressure fluid (vapor) having introduced into the mold cavity. The shape, the position and the size of the projections could not be controlled in the method. The projections have no meaning in planning the ribs of the resin moldings to be produced therein, but are rather problematic for the surface appearances of the moldings as causing sinkmarks in the surface of the moldings and even surface gloss unevenness of the moldings.
(3) To prevent the formation of the projections, still another method has been proposed, for which is used a low-foaming thermoplastic resin to be molded. In the method, a melt of the resin is injected into a mold cavity, then the cavity volume is enlarged in the primary stage, and thereafter the cavity volume is further enlarged in the secondary stage with an inert pressure fluid being introduced into the cavity under pressure. In this, a hollow space of the molding produced is formed in the secondary stage of cavity volume expansion. (See Japanese Patent Laid-Open No. 11151/1996.) However, the methods (1) and (2) noted above are still problematic in that the properties of the moldings obtained in those methods depend on the degree of reduction in their weight (that is, the degree of expansion in producing them) and on their shape. For example, some moldings having a large surface area (that is, large-surface moldings) often have low flexural strength and poor stiffness, and the moldings materials for them are specifically limited. Some others having a much reduced weight, for example those having been expanded to a degree of blow ratio of larger than 2 will often have poor flexural deformation resistance and poor flexural stiffness when they are so constructed that their inner structure is to have a uniform porosity. This is because, in the moldings of that type, the density of the center area in the thickness direction is greatly lowered. In the method (3), lightweight moldings having no surface projections and having good surface condition are obtainable. However, in the moldings having a relatively large surface area obtained in the method, the hollow space between the opposite surfaces will negatively act on the physical properties of the moldings, and, as a result, the moldings could not often have good stiffness and high strength. In addition, the method requires a relatively large amount of a foaming agent to be added to the molding resin materials, and such a large amount of the foaming agent added causes silver streaks to be formed in the surfaces of the moldings. What is more, in the method, the foaming degree of the foaming agent used must be controlled, and for this, the resin melt to be molded must be counter-pressured while it is injected into the mold cavity. For these reasons, the application of the method will be much limited. The object of the present invention is to provide lightweight resin moldings of which the advantages are that the latitude in selecting the molding materials to be used for them is broad, that the moldings have good stiffness and high strength even though they have a hollow space and have a large surface area, that the moldings have good appearances, that the latitude in controlling and reducing the weight of the moldings is broad, and that the moldings have many applications in various fields; and also to provide an efficient method for producing the moldings.
Given that situation as above, we, the present inventors have assiduously studied various starting resins for blow-injection molding and the moldings from them, and also the relationship between the reduction in the weight of the moldings and the inner structures and the physical properties of the weight-reduced moldings. As a result, we have found the following matters: In preferred inner structures of fiber-containing moldings, the pores do not form a hollow space but are substantially uniformly dispersed. However, for the moldings of which the weight is reduced to a higher degree, the molding conditions to form the preferred structures as above are limited to an extremely narrow range, and it is often difficult to stably form the intended structures. Therefore, depending on the molding conditions employed, the moldings could not often have high strength and good stiffness since their inner density, especially that in the center area in the thickness direction is often greatly lowered. For the moldings of which the weight is reduced to a higher degree, it is desirable to form a hollow space between the opposite surfaces of each molding and to form a reinforcing rib structure between them, and the moldings constructed in that manner can be stably produced on an industrial scale and have good stiffness and high strength.
We have further found the following matters: When a mold equipped with a rib-forming member is used, the intended lightweight resin moldings having good stiffness, high strength and good appearances can be produced efficiently. For producing-the moldings by the use of the mold of that type, a melt of a molding material that comprises a specific fiber-containing thermoplastic resin is injected into the mold cavity or is injected and compressed thereinto, then the mold cavity volume is expanded, and a gas is introduced into the resin melt after the start of the cavity expansion.
We have found that lightweight resin moldings having good stiffness, high strength and good appearances can be produced from fiber-containing, foamable molding materials by the use of the mold having the structure as above and by introducing a gas into the resin melt being molded. On the basis of these findings, we have completed the present invention.
Specifically, the invention provides the following:
1. A lightweight, hollow resin molding having a reinforcing rib structure in the hollow area, which is formed from a fiber-containing thermoplastic resin having a fiber content of from 10 to 70% by weight.
2. The lightweight resin molding of above 1, which has pores and of which the porosity including its hollow area falls between 25 and 95%.
3. The lightweight resin molding of above 2, wherein the pores are air-permeable ones.
4. The lightweight resin molding of above 3, wherein the air-permeable pores are dispersed.
5. The lightweight resin molding of above 1, wherein the fibers are glass fibers having a mean fiber length of from 0.2 to 20 mm.
6. The lightweight resin molding of above 1, wherein the fibers are glass fibers having a mean fiber length of from 2 to 15 mm.
7. The lightweight resin molding of above 1, wherein the reinforcing rib(s) is/are in the hollow area between the opposite surfaces of the molding.
8. A method for producing a lightweight, hollow resin molding having a reinforcing rib structure in the hollow area, from a fiber-containing thermoplastic resin having a fiber content of from 10 to 70% by weight, by the use of a mold equipped with a rib-forming member; the method comprising injecting a melt of a fiber-containing molding material which comprises a fiber-containing thermoplastic resin and in which the fibers have a mean fiber length of from 2 to 50 mm, into the mold cavity or injecting and compressing the melt thereinto, then expanding the mold cavity volume, and introducing a gas into the resin melt after the start of the cavity expansion.
9. The method for producing a lightweight resin molding of above 8, wherein the mold equipped with a rib-forming member is so constructed that its cavity except the rib-forming area is expandable.
10. The method for producing a lightweight resin molding of above 8, wherein the mold equipped with a rib-forming member is so constructed that at least one of the facing surfaces of the mold members is partially protruded or recessed.
11. The method for producing a lightweight resin molding of above 8, wherein the fiber-containing thermoplastic resin melt is prepared by plasticizing and melting fiber-containing thermoplastic resin pellets which have a total length of from 2 to 50 mm and contain fibers in an amount of from 20 to 80% by weight thereof, and in which the length of the fibers is the same as the total length of the pellets and the fibers are aligned in parallel to each other, or by plasticizing and melting a mixture of the fiber-containing resin pellets and other pellets of which the fiber content falls between 10 to 70% by weight thereof.
12. The method for producing a lightweight resin molding of above 8, wherein the speed at which the moving mold member is moved backward for expanding the mold cavity volume falls between 1 and 200 mm/sec.
13. The method for producing a lightweight resin molding of above 8, wherein a fiber-containing, foamable thermoplastic resin melt is injected into the mold cavity or is injected and compressed thereinto to thereby fill the mold cavity with the resin melt, then the mold cavity volume is expanded, and a gas is introduced into the resin melt after the start of the cavity expansion.
14. The method for producing a lightweight resin molding of above 8, wherein the melt of a fiber-containing molding material which comprises a fiber-containing thermoplastic resin and in which the fibers have a mean fiber length of from 2 to 50 mm contains from 0.5 to 10 parts by weight, relative to 100 parts by weight of the fiber-containing resin, of a foaming agent.
15. The method for producing a lightweight resin molding of above 8, wherein the expansion ratio of the mold cavity volume falls between 1.3 and 20.
16. The method for producing a lightweight resin molding of above 10, wherein a rib-corresponding part having a relatively higher density is formed between the surfaces of the mold members that face to each other in the cavity volume-expanding direction.
17. The method for producing a lightweight resin molding of above 8, wherein the cavity volume expansion is effected by moving a part of one mold member that forms the cavity.